1.1. Study site
The Limbe Botanical Garden (LBG) is the first botanical garden in Cameroon and the oldest in Africa. It was created in 1892, during the German colonial era, in Victoria (former name of Limbé), between the ocean and Mount Cameroon at 4°0'49.46″N and 9°12'3.13″E. Initially intended for agricultural purposes, it has become one of the main curiosities of the South West Cameroon Region. The Limbe Botanical Garden has also served as a training centre for Cameroonians in the fields of agriculture, horticulture and forestry. It is also an international centre for biodiversity research.
Today, the garden, which originally covered 250 hectares, has only 48 hectares, the rest (202 hectares) being the forestry part which is the subject of this study. The garden has about 1,500 taxa (1,000 herbaceous and 500 woody plants). There are rare or endangered plants : 150 endemics, 100 from the south-west, including Calamus sp, Prunus africana, Gnetum spp. Some plants are the object of particular attention, notably the African palms, the endemic plants of Mount Cameroon, the Musa spp. Others are cultivated for conservation purposes: Irvingia gabonensis, Garcinia kola, Afrostyrax kamerunensis, Cola spp, Prunus africana, Gnetum spp, Pterocarpus soyauxii, Diospyros, Rauvolfia vomitoria, Nauclea diderrichii, Terminalia spp, Enantia chlorantha, Eremomastax speciosa, Bryophyllum spp and Physostigma venenosum. The botanical garden also houses a herbarium, which in 2001 had about 21,000 specimens and more recently 30,000. Its acronym in the Index Herbariorum is SCA.
1.2. Data collection
For the flora inventory, we used plots of 100 m2 (10 m x 10 m). Twenty (20) surveys were set up in the forest of the Limbé botanical garden and the area of influence. Thus, on each plot, tree species were identified and their diameters (DBH ≥ 10 cm) were measured at 1.30 m above the ground. We noted information such as the number of adult and juvenile individuals. The species nomenclature is consistent with the different floras of Africa and Cameroon (Sosef et al., 2017 ; Willie et al., 2018 ; Qian, 2019). The Shannon-Weaver diversity index was used to measure the average amount of information given by the species indication of an individual in the collection. This average was calculated from the proportions of species found. This index represents the total information given by the frequency of the various species along the inventory area. This index is calculated as follows:
\({{H}}^{{{\prime }}}=-\sum _{\mathbf{i}=1}^{\mathbf{s}}\mathbf{P}\mathbf{i} \mathbf{I}\mathbf{o}\mathbf{g} \mathbf{P}\mathbf{i}\) with \({P}\left({i}\right)=\frac{{{n}}_{{i}}}{{N}}\)
where
H' = Shannon Biodiversity Index,
i = A species in the study area,
Pi = Proportional abundance or percentage importance of the species,
S = Total number of species,
ni = Number of individuals of a species in the sample,
N = Total number of individuals of all species in the sample.
Thus, the value of H' depends on the number of species present, their relative proportions, the sample size and the log base. The Shannon index ranges from 0 to log S.
Structural parameters and estimation of above-ground plant biomass
Structural parameters (density, basal area) were calculated to account for the structure of the garden vegetation. The above-ground plant biomass was estimated. In order to do this, faced with the limitations of the direct method (Walker et al., 2011), namely, the execution time, the cost of operations and the destruction of tree stems, we opted for allometric methods. The equation of Chave et al., (2005) was used. It is given by the following mathematical equation:
In this formula,
AGB is the aboveground biomass determined in kg;
DBH, the diameter at breast height or 1.30 m above ground level (in cm);
⍴, the anhydrous specific gravity of wood (in g/cm3);
ln, the natural logarithm and
exp, the exponential function.
For specific densities (⍴), we used the reference list of Reyes et al. When a species had several values, we used the mean value. For species for which we do not have the specific density, the default value (default ρ = 0.58 g/cm3) for African tropical forests was used as recommended by Djomo et al., 2017. The sequestered carbon rate is estimated at 50% of the biomass (IPCC, 2019). Finally, the mass of carbon dioxide (CO2) is obtained by multiplying that of carbon by 0.27.
Data analysis
The data collected from the floristic inventory were analysed qualitatively and quantitatively to assess floristic richness, composition and diversity, Sorensen's similarity coefficient (Sorensen 1948) and Shannon's diversity index (Shannon 1948).
For the comparison of the means of the structural parameters between the park forest and the LBG, we used the parametric Student's t test. To test the existence of possible relationships between some structural parameters of Cedrela odorata and those of other species, we used Pearson's r correlation. These tests were carried out using the R software version 2020 for Windows